This invention relates generally to circuit breaker control circuits, and, more particularly, to a backup direct current power supply and trip control circuit combination for opening an electronically controlled breaker or recloser upon the occurrence of a line overcurrent condition in the event that the voltage of the normal direct current power supply is less than the minimum operating voltage of the normal electronic trip control circuit.
As is known, the electronic trip control circuit of a high power circuit breaker generally requires a source of constant potential direct current energy. Typically this source is also used to supply power to the trip coil of the circuit breaker. In the event this direct current source fails while the circuit breaker is closed, the distribution line on the load side of the breaker is not adequately protected should an overcurrent or fault condition occur. To remedy this potential problem, a secondary direct current power source is provided in addition to the normally, used direct current source. In such systems, undervoltage relays are employed to connect this secondary direct current power source to the circuit breaker trip control circuits whenever the voltage of the normal direct current power supply falls below the minimum operating voltage required by the circuit breaker trip control circuits. In some systems, the preferred source is the rectified and regulated output of a control transformer having a primary winding coupled to one of the high voltage distribution lines, and the secondary direct current source is a station battery. However, of course, secondary direct current source must be properly maintained or failure of the normal direct current supply may occur when the secondary direct current supply is inoperative. In this event, the distribution line on the load side of a circuit breaker is not protected against overload or fault conditions. Therefore, it is an object of the present invention to provide a back-up trip circuit which, upon the failure of the preferred and secondary DC power sources, detects a line overcurrent condition, and utilizes line current to provide the direct current power required to trip the breaker.
U.S. Pat. No. 3,590,325, issued on June 29, 1971 to McMillen, discloses a circuit for sensing an undervoltage condition of the circuit breaker control power supply. Upon sensing this low voltage condition, a voltage sensitive relay causes the main circuit breaker to open (if it is closed) using current from a previously charged capacitor. Concurrently, the voltage sensitive relay disconnects the main power supply from the spring released closing coil of the circuit breaker, to prevent the circuit breaker from being closed while the voltage of the normal control circuit power supply is below its minimum operating level. As a result, the described circuit protects the distribution line on the load side of the circuit breaker against overcurrent or fault conditions which may occur while the circuit breaker control power supply is inoperative. While the described system provides improvement, many unnecessary interruptions of the distribution line can result. Therefore, it is another object of the present invention to disclose a backup trip circuit which, upon the failure of the normal direct current power supply, will trip the circuit breaker only when an overcurrent or fault condition occurs on the distribution lines supplied by the breaker.
In other electronically controlled circuit breakers such as those described in U.S. Pat. No. 4,027,203, issued May 31, 1977, to Moran et al, and U.S. Pat. No. 3,803,455, issued Apr. 9, 1974, to Willard, the normal direct current supply for the breaker trip coil and the breaker trip control circuit is generated by the current flowing in the distribution line on the load side of the breaker. In the systems shown, individual current transformers are coupled to each phase of the distribution line to be protected from overcurrents. The secondary current of each current transformer is rectified by individual single phase, full wave, rectifier bridge which charge a power supply capacitor. The capacitor voltage is maintained nearly constant by a parallel zener diode. Once the capacitor is fully charged, it provides a constant voltage, direct current supply for the circuit breaker control circuits. The latter are also coupled to the current transformers in order to sense distribution line overcurrents. Should a line overcurrent condition occur, the power supply capacitor is discharged through the trip coil of the circuit breaker. Once the open breaker is reclosed, the power supply capacitor must be recharged before another trip operation can be initiated. The time required to trip the circuit breaker when it is closed into a fault is therefor greater than the time required to trip the circuit breaker when the power supply capacitor has been fully charged. It is therefor a further object of the present invention to provide a system in which the backup trip circuit only becomes operative upon the failure of a normal, constant voltage DC power source, wherein the circuit breaker can be closed only when the voltage level of the normal direct current source reaches a selected minimum level.